Magnetic record/reproduce heads intended for use at frequencies as high as several hundred megahertz (MHz) are advantageously made of very thin films of magnetic material laminated onto a non-magnetic substrate (typically ceramic). These parts are then bonded together into a precisely polished and dimensioned head assembly. Using well known process steps, a suitable magnetic material, such as a metal alloy of FeAlSi (known as "Sendust"), is deposited by film deposition in very thin layers onto a thin substrate of a tough non-magnetic material, such as a small highly polished piece of ceramic. After a suitable number of layers of the magnetic metal have been deposited onto the substrate, a small piece of the substrate and its magnetic metal layers are bonded to another similar piece with the magnetic layers sandwiched between thin outer pieces of the substrate. The pieces so joined are precisely cut and polished to a mirror-like finish along a face at right angles to the magnetic layers. They are then bonded together with a non-magnetic insulating material such as silica (SiO.sub.2) so that the faces are closely and precisely aligned opposite each other. These opposed faces thus form a high efficiency magnetic "air gap" for the head. In order to obtain very high frequency response (e.g., 150 MHz and higher) and good magnetic and electrical efficiency, important dimensions of the head, such as the depth of the air gap and thicknesses of the magnetic layers, must be very small and extremely precise (e.g., measured in microns). The outer surface of the magnetic head where it bears against a magnetic recording medium (e.g., a magnetic tape) is highly polished to near perfect smoothness and is very small in length and width. The wear-resistance and other characteristics of this outer surface are very important.
In the course of manufacturing a thin-film magnetic head such as just described, the magnetic metal layers of the core and the ceramic substrate are subjected to elevated temperatures of the order of 500.degree. C. This necessarily occurs during the annealing of the magnetic core and during a step of bonding together parts of the head with molten SiO.sub.2. Because it is necessary to subject the metal and ceramic portions of the head assembly to such a large temperature excursion (e.g., between about 500.degree. C. and ambient), the thermal coefficient of expansion of the ceramic material used must be suitably matched to that of the metal of the magnetic core. If their respective thermal coefficients are enough different the ceramic and metal parts of the head will delaminate upon heating and/or subsequent cooling to ambient temperature. This requirement of suitably matched thermal coefficients imposes a significant restriction on the choice of a ceramic for the head assembly.
The toughness and the cutting, grinding and polishing characteristics of a ceramic are also important factors in selecting this material for use in a magnetic head such as described above. If the ceramic material is prone to be chipped, cracked or even microscopically fractured during the machining and processing operations in fabricating the ceramic and metal core into a magnetic head, a substantial number of heads may be ruined. Such defects due to the ceramic may occur at various stages of manufacture and these defects add considerably to the cost of the overall head manufacturing process. Moreover, because latent defects are difficult to detect, the "average" performance level of heads accepted as "good" is, in fact, somewhat degraded. It is highly desirable therefor that the ceramic material used in a head have good machining properties, that it be tough and fracture resistant, and that it be free of voids and stresses when finished.
In certain applications where the magnetic head is deliberately kept slightly separated from the magnetic medium in a record/reproduce system (e.g., a hard disk read-write data system typically used in computers), physical wear of the magnetic head is not a problem. Because the head not being in contact with the magnetic medium, relative motion between the two does not induce wear of the head. But in those applications where the head is necessarily in contact with the magnetic medium, such as in a tape recording system intended for use at frequencies as high as several hundred MHz, physical wear of the head becomes a major concern. The tape and head in such a system are moved relatively past each at very high speed (e.g., as high as 75 m/sec.). Because of aerodynamic factors and the need to keep the head in contact with the tape, a small positive pressure is applied between head and tape to keep them in intimate contact. The surface of the tape, which is typically formulated of a magnetic oxide, acts on the head as a buffing and lapping medium. Even though the ceramic of which the head is made is hard relative to most materials, it nonetheless is gradually worn away by the continual rubbing action of the tape. The kind of wear, whether the wear is "fracture" or "deformation" controlled, and the rate at which wear occurs are important factors in determining the useful life of the magnetic head. By way of example, a head with a ceramic body of MnNi oxide (widely used in the industry under the designation "MN-130") and a laminated metal core of "Sendust" can be worn-away to the point of catastrophic head failure in as little as 800 hours. The rate and degree of wear depends also upon such factors as the type of tape used in a system, the relative speed of the tape past the head, and the small amount of pressure needed to keep the head in contact with the tape. Moreover, differential wear of the magnetic core relative to the ceramic body can result in a condition termed "washout" (to be explained in detail hereinafter) which results in progressive loss of sensitivity of the head. Thus the wear characteristics of such a magnetic head are very important.
It is highly desirable, both from the standpoint of cost and of system efficiency, that a magnetic head have a service life considerably longer than 800 hours. The present invention provides an efficient and cost effective solution to the problems and limitations with a magnetic head, and with its ceramic body, such as described above.